The Etiquette of the Teacher by Imam Nawawi

1, that his teachings are exemplary of true Ikhlass (sincerity, purity)

Sheikh al Hashimi
once went to teach a lesson that he was supposed to teach in the Umayyad mosque in Damascus and no one came, so he opened his book and read the lesson aloud and when he finished he closed it and left. His intention to teach was for Allah Subhanawataala and no more.

2. CONTINUOUSLY REFRESH KNOWLEDGE…

One should remain occupied in your knowledge; one should stay engaged in knowledge.

Many Imams are engaged in their many duties and cannot enrich themselves with the knowledge.